This invention relates to the preparation of polyesters, and more particularly to the preparation of poly(1,4-cyclohexanedicarboxylates).
Poly(1,4-cyclohexanedicarboxylates) such as poly(1,4-cyclohexanedimethyl-1,4-cyclohexanedicarboxylate), hereinafter sometimes designated "PCCD" for brevity, are known polyesters; reference is made, for example, to U.S. Pat. No. 2,891,930. PCCD is characterized by such advantageous properties as crystallinity and resistance to weathering under conditions of exposure to ultraviolet radiation. These properties have increased interest in commercialization in recent years.
However, the crystallinity of PCCD is not as high as that of such commercially available polyesters as poly(1,4-butylene terephthalate), or "PBT". While the crystalline melting temperature, Tm, of PCCD is in the range of 220-235.degree. C., a relatively high figure, its temperature of crystallization from the melt, Tc, is frequently on the order of 152-171.degree. C. Therefore, it crystallizes only slowly and its solvent resistance is not as high as might be desired. Experience has shown that rapid crystallization is exhibited by polymers having a minimum value of the parameter Tm-Tc (i.e., Tm minus Tc).
In the preparation of such polyesters as PBT from glycols and dialkyl terephthalates such as dimethyl terephthalate, it is customary to employ a stoichiometric excess of the volatile glycol (e.g., ethylene glycol or 1,4-butanediol) on the order of 20-50% and remove the excess by distillation after a first oligomerization or "pre-condensation" step, proceeding then to an "ester interchange" step of molecular weight building. This is, however, not possible in PCCD preparation since the diol employed, 1,4-cyclohexanedimethanol (hereinafter sometimes "CHDM"), is relatively non-volatile (boiling point .apprxeq.283.degree. C.). Instead, it has been necessary to utilize a one-step process for the reaction of CHDM with 1,4-cyclohexanedicarboxylic acid esters such as the dimethyl ester (hereinafter sometimes "DMCD") and exercise tight control of stoichiometry to produce a high molecular weight product (most often a weight average molecular weight, Mw, of 70,000 or greater). For example, DMCD is typically employed in about a 0.5 mole percent excess. This requires the imposition of strict limitations on the process conditions.
Various factors have been found to affect the crystallinity of the product. They include its isomeric state and the isomeric states of the reagents, and the tendency of crosslinking and/or branching reactions to occur during polymerization.
The proportion of trans isomer in DMCD as typically supplied is on the order of 99%. During the PCCD-forming reaction, isomerization can take place with an increase in the level of the cis isomer, decreasing crystallinity. The CHDM reagent is normally about 70% trans, but no isomerization of those molecules takes place during product formation.
Also, it has been observed that some degree of gel formation occurs during polymerization, decreasing the value of Tc and thus the crystallinity level. This is apparently the result of crosslinking-branching reactions.
It would be desirable to develop a preparation method for poly(1,4-cyclohexanedicarboxylates) which is not as subject to process limitations and can nevertheless produce a high molecular weight polymer. It would also be desirable to produce polymers which crystallize rapidly from the melt.